Studies on energy efficiency in vehicles require as input data values for the drag (Cd) and the rolling resistance (Fr) coefficients obtained under real operational conditions of the vehicles. Frequently, these values are unknown for the case of cargo and passenger transportation vehicles, which are the largest consumers of fuel among ground vehicles. The accepted measurement methods for the determination of Cd and Fr are expensive and are still being questioned for their low capacity to reflect the real operational conditions of the vehicles. This work evaluates the level of accuracy and precision of Cd and Fr obtained by on road coast down tests, which is an easy to implement and affordable alternative method to obtain those coefficients under real operational conditions. We added to this method the influence of the road grade and the use of the generalized reduced gradient technique to minimize the differences between the speed obtained by an analytical model, where Cd and Fr are an unknown, and the instantaneous speed measured during cost down tests conducted following the testing protocols recommended by the SAE J1263. This combined method was applied to a sample of 19 vehicles. The obtained values of Cd and Fr showed an average accuracy error < 5.2 % and an average precision error < 6.1%, that are acceptable for studies in energy efficiency in vehicles.